Magnetic recording medium having an underlayer containing three different powders

ABSTRACT

A magnetic recording medium containing a support having thereon in order, a non-magnetic layer unit and a magnetic layer unit, said magnetic recording medium having been produced by providing a non-magnetic layer unit containing at least one non-magnetic layer and comprising a non-magnetic powder dispersed in a binder on a non-magnetic support followed by providing, while the non-magnetic layer is still wet, a magnetic layer unit containing at least one magnetic layer and comprising a ferromagnetic powder dispersed in a binder on the non-magnetic layer, wherein the mean thickness of the magnetic layer is 1.0 μm or less and the non-magnetic powder in the non-magnetic layer comprises (A) a tabular non-magnetic powder having a tabular size of from 0.01 to 0.3 μm and an aspect ratio of from 3 to 20, or an acicular non-magnetic powder having a long axis of from 0.05 to 0.25 μm and an aspect ratio of from 3 to 15, (B) a carbon black powder having a mean particle size of from 0.01 to 0.04 μm and (C) a powder component having a larger mean particle size than the mean particle size of either powders (A) or (B).

This is a Continuation-In-Part of application Ser. No. 08/010,846, filedJan. 29, 1993, now U.S. Pat. No. 5,455,112.

FIELD OF THE INVENTION

The present invention relates to a magnetic recording medium, especiallyto one having an extremely thin magnetic layer having a mean thicknessof 1.0 μm or less. More precisely, it relates to a coating type magneticrecording medium having excellent running property.

BACKGROUND OF THE INVENTION

Magnetic recording media are widely used as a sound recording tape,video tape, computer tape, recording disc and the like. Ever-increasingdemands have had to be met for increased magnetic recording density andshortened (higher frequency) recording wavelength. The recording systemfor these devices varies from an analogue system to a digital system. Inresponse to the current demand for elevation of the magnetic recordingdensity of the recording medium, a magnetic recording medium having athin metal film, e.g., vapor-deposited, as the magnetic layer has beenproposed. However, in view of the more facile productivity and practicalreliability against corrosion or the like, a so-called coating typemagnetic recording medium is nonetheless preferred. In a coating typemagnetic recording medium, a dispersion of a ferromagnetic powder in abinder has been coated on the support. However, since a coated mediumhas a relatively low filling degree with respect to the magneticsubstance, as compared with a medium having a thin metal film, theformer is inferior to the latter with respect to the electromagneticcharacteristic.

In any event, a widely used coating type magnetic recording mediumcomprises a dispersion of a ferromagnetic iron oxide, Co-modifiedferromagnetic iron oxide powder, CrO₂ powder or ferromagnetic alloypowder that has been coated on a non-magnetic support to form a magneticlayer thereon.

In order to improve the electromagnetic characteristic of such a coatingtype magnetic recording medium proposals have included, for example,improvement of the magnetic characteristic of the ferromagnetic powderadded to the magnetic layer of the medium and smoothing of the surfaceof the medium. However, these proposed methods are not adequate tosufficiently elevate the magnetic recording density of the medium.

Recently, the recording wavelength for a coating type magnetic recordingmedium is being shortened concomitantly with elevation of the magneticrecording density of the medium. As a result, if the thickness of thecoated magnetic layer is large, problems of self-demagnetization loss inrecording with a lowered output arise and thickness loss in reproductionare serious.

Therefore, in response, reduction in the thickness of the magnetic layerhas been attempted. If, however, the thickness of the magnetic layer isreduced to about 2 μm or less, the surface of the magnetic layer wouldoften be influenced by the non-magnetic support so that theelectromagnetic characteristic and drop-out of the medium would worsen.However, the influence of the rough surface of the support would beavoided if a thick non-magnetic undercoating layer is provided on thesurface of the support and then a magnetic layer is coated over theundercoating layer as an upper layer, as proposed in JP-A 57-198536 (theterm "JP-A" as used herein means an "unexamined published Japanesepatent application"). However, that method still has a problem in thatthe head abrasion resistance and the head durability worsen. The problemis considered attributable to the fact that a thermosetting resin isused as a binder in the non-magnetic lower layer. As a consequence, thelower layer is hardened so that the friction between the magnetic layerand head, as well as the contact of the magnetic layer with other parts,are effected under no buffer condition. The magnetic recording layerhaving such a lower layer has poor flexibility.

In order to avoid the problem, the use of a non-hardening resin as abinder in the lower layer is conceivable.

In accordance with the conventional method where the lower layer iscoated and dried and then the magnetic layer is coated thereover as anupper layer, however, the lower layer would be swollen by the organicsolvent in the coating solution for the upper layer to cause turbulenceof the coating solution for the upper layer. As a result, the surfaceproperty of the magnetic layer would thereby be worsened and theelectromagnetic characteristic of the medium would thereby be lowered.However, in order to reduce the thickness of the magnetic layer,reduction of the amount of the magnetic coating solution for the upperlayer, or reduction of the concentration of the magnetic coatingsolution by adding a large amount of a solvent thereto, may beenvisioned. In the former case of reducing the amount of the coatingsolution for the magnetic coating layer, however, drying of the coatedlayer would start before allowing sufficient leveling time to pass afterthe coating to cause another problem of coating defects of givingstreaks or stamped patterns on the surface of the magnetic layer coated.As a result, the yield of the method would be extremely low.

On the other hand, in the latter case of using a magnetic coatingsolution having a low concentration, the coated film would be highlyporous so that sufficient filling degree of a magnetic substance in thefilm could not be attained,. and additionally, the strength of thecoated film would be insufficient as the film is too porous. In short,both methods mentioned above have various unfavorable problems. Also, inthe invention of JP-A-62-154225, poor yield is a serious problem leftunresolved.

The present applicant has already proposed, as one means of overcomingthe above problems, a method of simultaneous multi-coating system wherea non-magnetic lower layer is provided and, while the lower layer isstill wet, an upper magnetic layer containing a ferromagnetic powder isprovided over the lower layer, as described in JP-A-63-191315(corresponding to U.S. Pat. No. 4,963,433) and JP-A-63-187418(corresponding to U.S. Pat. No. 4,863,493). This technique yields amagnetic recording medium with high producibility, wherein the medium isfree from coating defects and has elevated productivity, improvedelectromagnetic characteristics such as output and C/N ratio andimproved running durability.

In accordance with the conventional method, however, it is stilldifficult to satisfy the current demand for a high-recording densitymagnetic recording medium having a thin coated layer suited for extendeduse, in particular, for such a device having a thin magnetic layerhaving a thickness of 1 μm or less. That is, if such a thinhigh-recording density magnetic recording medium is prepared by theconventional method, the medium has coating defects and another problemthat both the electromagnetic characteristic and the running durabilitycould not be satisfied well. In particular, when the running durabilityof a thin tape is desired to be increased, reduction of damage of thetape edge is necessary. However, the conventional method could notsatisfy the demands.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a magnetic recordingmedium having good electromagnetic characteristics and having excellentrunning durability with little edge damage after repeated running.

Specifically, in order to attain this object, the present inventionprovides a magnetic recording medium produced by providing a lowernon-magnetic layer comprising a non-magnetic powder dispersed in abinder on a non-magnetic support followed by providing, while the lowernon-magnetic layer is still wet, an upper magnetic layer comprising aferromagnetic powder dispersed in a binder on the lower non-magneticlayer, wherein the mean thickness of the upper magnetic layer is 1.0 μmor less and the non-magnetic powder in the lower non-magnetic layercomprises (A) a granular inorganic powder having a mean particle size offrom 0.01 to 0.08 μm, (B) a carbon black powder having a mean particlesize of from 0.01 to 0.04 μm and (C) a powder component having largermean particle size than the mean particle size of each of powders (A)and (B).

DETAILED DESCRIPTION OF THE INVENTION

In producing a magnetic recording medium in accordance with the presentinvention, a lower non-magnetic layer (hereinafter referred to as a"non-magnetic layer" or a "lower layer") is provided on a non-magneticsupport and, while the non-magnetic layer is still wet, an uppermagnetic layer (hereinafter referred to as a "magnetic layer" or an"upper layer") is provided over the non-magnetic layer so that the yieldof production on the medium is elevated. Specifically, the meanthickness of the magnetic layer of the medium of the present inventionis defined to be 1 μm or less so that self-demagnetization may bereduced and the output to be applied to the medium may be elevated.

In addition, the dispersibility of the non-magnetic layer may beelevated and the surface smoothness of the layer may be improved byadding the granular inorganic powder (A) to the layer. However, if thecontent of (A) in the non-magnetic layer is too great, the porosity ofthe layer would be small so that the layer would be too dense andbrittle and, as a result, the layer would have poor electroconductivity.Due to the small porosity, the layer would not be effective tosupplement of a lubricating agent thereto. However, by addition of thecarbon black (B), the layer may have suitable porosity andelectroconductivity.

A thin tape often suffers from edge damage. However, by addition of thecoarse third component powder (C) to the layer, the edges of the tapemay have suitable projections and depressions to be suitably rough sothat the friction resistance of the edges thereof may be reduced. As aresult, a high output of 7 MHz may be applied to the medium of thepresent invention, and additionally, the medium is improved to have highC/N characteristic, excellent still durability and excellent runningdurability.

In accordance with the present invention, the powder composition of thelower non-magnetic layer of the recording magnetic medium has beenspecifically defined so that the dynamic characteristic of the magneticrecording medium is controlled to inhibit edge damage of the magneticrecording medium and additionally the electromagnetic characteristic ofthe medium in short-wavelength recording is improved.

Production of the medium of the present invention is characterized inthat the kind of the non-magnetic powder used in the coating solutionfor the lower non-magnetic layer is specifically selected and used inorder that an upper magnetic layer having an extremely small drythickness, especially having a dry thickness of 1 μm or less, may beprovided on the lower non-magnetic layer with no coating defects andthat the upper layer is over-coated over the lower layer previouslycoated on a non-magnetic support while the lower layer on the support isstill wet. Accordingly, the present invention provides a magneticrecording medium suited for industrial mass-production having no coatingdefects such as pin holes or streaks, and the medium has an extremelythin magnetic layer having a capability comparable to a ferromagneticmetal layer, and having excellent running durability with little edgedamage.

For the purpose of better ensuring the excellent electromagneticcharacteristic and the improved running durability of the magneticrecording medium of the present invention, the size of the constitutivepowders (A), (B) and (C) and the kind and shape of materials of them arespecifically selected and defined.

Precisely, the non-magnetic powder to be in the lower non-magnetic layerof the medium of the present invention comprises at least a granularinorganic powder (Powder A) having a mean particle size of from 0.01 to0.08 μm, preferably from 0.02 to 0.06 μm, a carbon black powder (PowderB) having a mean particle size of from 0.01 to 0.04 μm, preferably from0.015 to 0.03 μm, and a third component powder (Powder C) which as alarger mean particle size than powder (A) and the powder (B).

The powder (A) may be granular, which means that the particlesconstituting it are such that the ratio (long axis/short axis) in thelength of the two axes, as freely selected from the constitutingparticles, falls within the range of generally from 0.6 to 1.4,preferably from 0.8 to 1.2. Specifically, it means that tabular oracicular particles are excluded from the powder (A) when it is granular.If the powder (A) is not granular, the dispersibility of the coatingsolution would be poor so that, when the solution is coated, a smoothsurface of a coated layer may not be obtained.

Nevertheless, powder (A) may alternatively be acicular or tabular.

When the powder (A) is an acicular non-magnetic powder, the powder has along axis of from 0.05 to 0.25 μm, preferably from 0.065 to 0.22 μm, anaspect ratio of from 3 to 15, preferably from 4 to 10, and a specificsurface area by BET method of preferably from 40 to 65 m² /g, morepreferably from 45 to 60 m² /g. The acicular non-magnetic powderpreferably mainly comprises α-Fe₂ O₃ or TiO₂.

When the powder (A) is a tabular non-magnetic powder, the powder has atabular size of from 0.01 to 0.3 μm, preferably from 0.02 to 0.2 μm, anaspect ratio of from 3 to 20, preferably from 3 to 10, and a specificsurface area by BET method of preferably from 40 to 65m² /g, morepreferably from 45 to 60 m² /g. The tabular non-magnetic powderpreferably mainly comprises α-Fe₂ O₃.

For purposes of this invention, the mean particle size of the powdersmeans either a mean value of the diameter of powder particles when thepowders have a granular or polyhedral shape, or a mean value of thelength of the longest axis of the powder particles when the powders havean acicular or tabular shape, as obtained by observation with atransmission electronic microscope. The same definition of mean particlesize shall apply to the other powders to be used in the presentinvention having powder shapes akin to any of the above-mentionedcategories.

The powder (C) is not specifically limited in kind or physical geometry,provided it has a larger mean particle size than the powder (B) and thepowder (A). Preferably, it is selected from the following categories ofmaterials (1) to (3). The powder (C) to be selected preferably has Mobs'hardness of generally 4 or more, preferably 5 or more, and a specificgravity of generally from 2 to 6, preferably from 3 to 5.

(1) Granular or polyhedral powder having a mean particle size of from0.07 μm to less than 1 μm:

The granular or polyhedral powder means to specifically exclude atabular or acicular powder. Examples of the granular or polyhedralpowder include a spherical powder or a regular polyhedral or irregularpolyhedral powder in which the constitutive plane of each particle isselected from one or more regular n-angles such as regular square,regular 5-angle and regular 6-angle or from one or more irregularn-angles. A preferred powder is one having a ratio (long axis/shortaxis) in the length of the freely selected two axes which is within therange of preferably from 0.6 to 1.4, more preferably from 0.8 to 1.2.The mean particle size of the powder is preferably within the range offrom 0.1 to 0.5 μm.

(2) Acicular powder having a mean length in the short axis of from 0.05μm to less than 1 μm and having an aspect ratio of 10 or more;

The aspect ratio means a ratio of mean long axis/mean short axis of eachparticle of this category. Preferred examples of the acicular powder ofthis kind include α-iron oxide, α-goethite, TiO₂, asbestos, ZnO,potassium titanate and silica.

More preferably, the mean short axis of the powder is from 0.1 to 0.5 μmand the aspect ratio thereof is from 10 to 50.

(3) Tabular powder having a tabular diameter of from 0.1 μm to less than2 μm and an aspect ratio of 10 or more:

The aspect ratio means a ratio of tabular diameter/tabular thickness ofeach particle of this category. Preferred examples of the tabular powderof this kind include graphite, talc powder, MIO (mica iron oxide),kaolin and clay.

More preferably, the mean tabular diameter of the powder is from 0.15 to1 μm and the aspect ratio thereof is from 10 to 100.

The above-mentioned powders (1) to (3) may be used singly or incombination of them as the powder (C) in the present invention.

In the present invention, the non-magnetic powder used in the lowernon-magnetic layer preferably comprises only the powder (A), the powder(B) and the powder (C), in which the volume proportion of the powder (A)is desired to be from 40 to 80%, preferably from 45 to 75%, that of thepowder (B) is from 15 to 40%, preferably from 25 to 35%, and that of thepowder (C) is from 2 to 26%, preferably from 5 to 20%. The volumeproportion means the proportion of each powder component to the totalvolume of the non-magnetic powder in the lower layer. Specifically, thesum of the respective volume proportions of the powder (A), the powder(B) and the powder (C) is preferred to represent 100% of the totalvolume of non-magnetic powder in the lower layer. If desired, however,the non-magnetic powder may additionally contain any other inorganic ororganic non-magnetic powder component in a range of attaining the objectof the present invention, generally in a range of 15% or less.

The powder volume content of the non-magnetic powder of the lower layeris preferably from 20 to 60%, especially preferably from 22 to 50%,based on the total volume of the lower layer.

In order to improve the electromagnetic characteristics of the magneticrecording medium of the present invention, it is desired that thedispersibility of the coating solution for the lower layer is good andthat the surface smoothness of the over-coated layer is good. For thispurpose, the non-magnetic powder for the lower layer is desired to havea large content of the powder (A). This is because the powder (A) has asmaller specific surface area than the powder (B), and thus powder (A)has less adhesion of powder particles to each other than the powder (B).Therefore, powder (A) has a greater influence on the dispersibility ofthe non-magnetic powder solution for the lower layer.

However, if the amount of powder (A) is too great and the amount ofpowder (B) is too small, the lower layer formed would have poor porosityand would dense, so that the magnetic recording medium having such adense lower layer would be brittle. As a result, the still durability ofthe medium would be insufficient, powder dropping from the medium wouldoften occur, the electroconductivity of the medium would be poor, andthe medium would often suffer from running hindrance under electriccharge. Additionally, since the porosity of lower layer would be low,supplements of a lubricant thereto would be insufficient so that thestill durability of the medium would worsen. Under this situation, inorder to ensure suitable porosity and electroconductivity of the lowerlayer, the amount of the powder (B) in the lower layer is increased.However, if the content of the powder (B) is too great, thedispersibility of the powder solution of forming the lower would beinsufficient and the shrinkage of the lower layer would noticeablydiffer from that of the upper magnetic layer to cause curling of themedium having them.

By suitably admixing the powder (A) and the powder (B), the problem maybe buffered. However, using only the powder (A) and the powder (B), evenin a judicious manner, are not satisfactory for attaining sufficientrunning durability of the medium to often cause edge damage especiallyin a thin tape having a medium thickness of 15 μm or less.

Therefore, the powder (C) is added to the lower layer so as to solvethis problem. Due to addition of the powder (C), it is believed that theedges of a tape with the medium may have suitable projections anddepressions to be properly roughened so that the friction resistance ofthe edges may thereby be reduced by the function of powder (C).

Therefore, the kind and the size of the constitutive powders of thenon-magnetic powder for the lower layer are defined in the presentinvention as mentioned above. In addition, the volume proportion of theconstitutive powders is also defined in the manner as mentioned above,whereby the electromagnetic characteristic, the still durability and therunning durability of the medium of the present invention are all wellsatisfied.

Therefore, the powder (C) preferably should have the volume proportionfalling within the range as defined above. If the volume proportion ofthe powder (C) is too high, the dispersibility of the non-magneticpowder composition for the lower layer would worsen to lower the surfacesmoothness of the medium and, as a result, the electromagneticcharacteristic of the medium is thereby lowered. On the contrary, if thevolume proportion of the powder (C) is too small, powder dropping fromthe edges of a tape with the medium would be great so that the runningdurability could not be improved.

In the medium of the present invention, the powder (C) is desired to beexposed from the surfaces of the layer in some degree. Therefore, thethickness of the lower layer needs to be somewhat large. This thicknesspreferably is within the range of from 1 to 5 μm and particularly from1.5 to 3.5 μm. If the thickness is less than about 1 μm, the powder (C)is overly exposed as protrusions of the overlying magnetic layer so thatthe surface property of the medium is unduly roughened. However, if thethickness is too large, adhesion of the powder (C) is lowered to cause aproblem of powder dropping.

For example, the granular of polyhedral powder for the powder (A) andthe powder (C), category (1), to be used in the lower non-magnetic layerof the medium of the present invention is selected from the groupconsisting of non-magnetic inorganic powders of metals, metal oxides,metal carbonates, metal sulfates, metal nitrides, metal carbides andmetal sulfides.

Examples of such granular or polyhedral powders for (A) and (C) includeTiO₂ (rutile, anatase), TiOx, cerium oxide, tin oxide, tungsten oxide,ZnO, ZrO₂, SiO₂, Cr₂ O₃, α-alumina with alphatization of 90% or more,β-alumina, γ-alumina, α-iron oxide, goethite, corundum, silicon nitride,titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide,copper oxide, MgCO₃, CaCO₃, BaCO₃, SrCO₃, CaSO₄, BASO₄, silicon carbideand titanium carbide. These powders are selected and used, singly or incombination of two or more of them, in accordance with theabove-mentioned conditions. As powder (A), preferred are titanium oxide,barium sulfate, silica, alumina, zinc oxide and α-iron oxide. Asexamples of granular or polyhedral powder for powder (C), α-alumina withalphatization of 90% of more, α-alumina, γ-alumina, γ-alumina, α-ironoxide, TiO₂ (rutile, anatase), CeO₂, SnO₂, ZnO, ZrO₂, SiC, TiC, SiO₂,Cr₂ O₃, BN, MoS₂ and WO₂ are preferred.

As the inorganic powders mentioned above, those having the propertiesmentioned below are preferred. Preferred powders are those having a tapdensity (measured under the condition of numbers of tapping: 150 andhead: 30 mm, using Tap density measuring instrument manufactured bySeishin Enterprise Inc.) of from 0.05 to 2 g/ml, preferably from 0.2 to1.5 g/ml; a water content of from 0.1 to 5%, preferably from 0.2 to 3%;a pH value of from 2 to 11; a specific surface area of from 1 to 100 m²/g, preferably from 5 to 50 m² /g, especially preferably from 7 to 40 m²/g; a crystal unit size of from 0.01 μm to 2 μm; an oil absorption withDBP of from 10 to 100 ml/100 g, preferably from 15 to 80 ml/100 g, morepreferably from 20 to 60 ml/100 g; and a specific gravity of from 2 to8, preferably from 3 to 6.

The above-mentioned inorganic powders need not necessarily be 100% pure.For instance, in accordance with the object desired, the surfaces ofthese powders may be treated with other compounds such as compounds ofAl, Si, Ti, Zr, Sn, Sb and/or Zn to form oxides thereof on the surfacesthereof. In this case, the content of the substrate powders can be 70%or more so as not to reduce the effect of them. For instance, whentitanium oxide is used as the powder, the surface is generally treatedwith alumina. The ignition loss of the powders is desired to be 20% orless. The Mohs' hardness is desired to be 3 or more.

Specific examples of the inorganic powder to be used in the presentinvention include UA5600 and UA5605 manufactured by Showa Denko K.K.;AKP-20, AKP-30, AKP-50, HIT-55, HIT-100 and ZA-G1 manufactured bySumitomo Chemical Co.; G5, G7 and S-1 manufactured by Nippon ChemicalIndustry Co.; TF-100, TF-120, TF-140 and R516 by Toda Kogyo Co., Ltd.;TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, FT-1000, FT-2000,FTL-100, FTL-200, M-1, S-1, SN-100, R-820, R-830, R-930, R-550, CR-50,CR-80, R-680 and TY-50 manufactured by Ishihara Sangyo Kaisha Ltd.;ECT-52, STT-4D, STT-30D, STT-30 and STT-65C manufactured by Titan KogyoK.K.; T-1 manufactured by Mitsubishi Material Co., Ltd.; NS-O, NS-3Y andNS-8Y manufactured by Nippon Shokubai Kagaku Kogyo Co., Ltd.; MT-100S,MT-100T, MT-150W, MT-500B, MT-600B and MT-100E manufactured by TeikaK.K.; FINEX-25, BF-1, BF-10, BF-20, BF-1L and BF-10P manufactured bySakai Chemical Industry Co., Ltd.; DEFIC-Y and DEFIC-R manufactured byDowa Mining Co., Ltd.; and Y-LOP manufactured by Titan Kogyo K.K.; andits calcined product.

As examples of powder (A), especially preferred is titanium oxide(particularly, titanium dioxide). A method of producing titanium oxidefor the powder (A) will be described in detail hereafter.

For producing titanium oxide, a sulfuric acid method and a chlorinemethod are mainly employed. In a sulfuric acid method, raw ores ofilmenite are distilled with sulfuric acid to extract Ti and Fe as theirsulfates; iron sulfate is removed by crystallization separation and theremaining titanyl sulfate solution is filtered and purified and thenhot-hydrolyzed to give titanium oxide hydrate precipitates; theprecipitates are filtered and washed and impurities are removedtherefrom; and a particle size adjusting agent is added to the purifiedprecipitates, which are then calcined at 80° to 1000° C. to give a crudetitanium oxide. The product is grouped into a rutile titanium oxide andanatase titanium oxide due to the kind of the nucleating material to beadded to the hydrolysis step. The crude titanium oxide is ground andtreated for dressing the resulting particles, which are thensurface-treated.

On the other hand, in a chlorine method, raw ores of natural futile orsynthetic rutile are chlorinated at a high temperature in a reducingatmosphere, whereby Ti is converted into TiCl₄ and Fe into FeCl₂, andthe cooled and solidified iron oxide is separated from the liquid TiCl₄.The thus-obtained crude TiCl₄ is purified by rectification, and anucleating agent is then added thereto and is reacted instantaneouslywith oxygen at a temperature of 1000° C. or higher to give a crudetitanium oxide. The finishing step of imparting a pigment-like propertyto the crude titanium oxide as obtained in the oxidative decompositionstep is the same as that in the above-mentioned sulfuric acid method.

The surface of the titanium oxide thus-obtained may be treated withvarious compounds as desired in the circumstances.

As powder (B), a carbon black powder is used in the present invention,which includes, for example, furnace black for rubber, thermal black forrubber, carbon black for coloration and acetylene black. It is preferredthat the powder (B) has a specific surface area of generally from 100 to500 m² /g, preferably from 150 to 400 m² /g, a DBP oil absorption ofgenerally from 20 to 400 ml/100 g, preferably from 30 to 200 ml/100 g, apH value of generally from 2 to 10, a water content of generally from0.1 to 10%, and a tap density of generally from 0.1 to 1 g/ml.

Specific examples of the powder (B) include BLACKPEARLES 2000, 1300,1000, 900, 800, 880, 700 and VULCAN XC-72 manufactured by Cabot Co.,Ltd.; #3050, #3150, #3250, #3750, #3950, #2400, #2300, #1000, #970,#950, #900, #850, #650, #40, MA40 and MA-600 manufactured by MitsubishiChemical Corporation; CONDUCTEX SC manufactured by Columbia Carbon Co.,Ltd.; 8800, 8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255and 1250 manufactured by RAVEN Co., Ltd.; and Ketjen Black ECmanufactured by Agzo Co., Ltd. The carbon black may be surface-treatedwith a dispersing agent or may be grafted with a resin to partiallygraphatize the surface of it. If desired, the carbon black maypreviously be dispersed in a binder prior to its addition to thenon-magnetic coating solution for the lower layer. The above-mentionedcarbon blacks may be used singly or as a mixture of two or more of them.

As specific examples of the carbon black described herein for use in thepresent invention, reference can be had to the Handbook of Carbon Black(published by Carbon Black Association).

If desired, a non-magnetic organic powder may be incorporated into thelower layer of the medium of the present invention. Examples of suitablenon-magnetic organic powders include an acryl-styrene resin powder, abenzoguanamine resin powder, a melamine resin powder and aphthalocyanine pigment powder. Also usable organic powders are apolyolefin resin powder, a polyester resin powder, a polyamide resinpowder, a polyimide resin powder, and a polyethylene fluoride resinpowder. For methods of producing these powders, JP-A-62-18564 andJP-A-60-255827 may be referred to.

The proportion of the inorganic and organic non-magnetic powder used inthe lower layer of the present invention is generally from 20 to 0.1 byweight and is from 10 to 0.1 by volume, based on the amount of binder inthe lower layer.

Provision of a conventional undercoating layer for a magnetic recordingmedium is also preferably effected. The purpose of the undercoatinglayer is to improve the adhesiveness between the support and theoverlying layers including the magnetic layer. The thickness of theundercoating layer is typically 0.5 μm or less. Therefore, theundercoating layer is different from the lower non-magnetic layer of themedium of the present invention. Also, in the magnetic recording mediumof the present invention, provision of such an undercoating layerdirectly on the support is preferred so as to enhance the adhesivenessbetween the lower non-magnetic layer and the support.

As the ferromagnetic powder to be used in the magnetic layer of themedium of the present invention, known ferromagnetic powders aresuitable, such as a magnetic iron oxide FeOx (x=1.33 to 1.5), aCo-modified FeOx (x=1.33 to 1.5), a ferromagnetic alloy powderconsisting essentially of Fe, Ni or Co (in an amount of 75% or more), abarium ferrite powder and a strontium ferrite powder. Of these,especially preferred is a ferromagnetic alloy powder. The ferromagneticpowder for use in the present invention may contain, in addition to theprincipal atoms, any other atoms of Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo,Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd,P, Co, Mn, Zn, Ni, Sr and/or B.

The ferromagnetic powder may previously be treated with a dispersingagent, a lubricant, a surfactant and/or an antistatic agent prior todispersion, as mentioned below. Such a pre-treatment is described in,for example, JP-B-44-14090, JP-B-45-18372, JP-B-47-22062, JP-B-47-22513,JP-B-46-28466, JP-B-46-38755, JP-B-47-4286, JP-B-47-12422,JP-B-47-17284, JP-B-47-18509, JP-B-47-18573, JP-B-39-10307,JP-B-48-39639 (the term "JP-B" as used herein means an "examinedJapanese patent publication"), and U.S. Pat. Nos. 3,026,215, 3,031,341,3,100,194, 3,242,005, 3,389,014.

Of the above-mentioned ferromagnetic powders, the ferromagnetic alloypowder may contain a small amount of hydroxide(s) and/or oxide(s). Theferromagnetic alloy powder for use in the present invention may beprepared by known methods, which are, for example, (a) a method ofreducing a composite organic acid salt (essentially, oxalates) with areducing gas such as hydrogen, (b) a method of reducing iron oxide witha reducing gas such as hydrogen to give Fe or Fe--Co particles, (c) amethod of thermal-decomposing a metal carbonyl compound, (d) a method ofadding a reducing agent such as sodium borohydride, a hydrophosphite orhydrazine to an aqueous solution of a ferromagnetic metal to reduce themetal, and (e) a method of vaporizing a metal in a low pressure inertgas to obtain a fine powder of the metal. The thus-obtainedferromagnetic alloy powder may be subjected to a known gradual oxidationtreatment, for example, by a method of dipping the powder in an organicsolvent followed by drying, a method of dipping the powder in an organicsolvent followed by applying an oxygen-containing gas so as to form anoxide film on the surface thereof and drying the coated powder, or amethod of forming an oxide film on the surface of the powder byadjusting the partial pressure of an oxygen gas and an inert gas asapplied to the powder without using an organic solvent.

The ferromagnetic powder of the upper magnetic layer of the medium ofthe present invention has a specific surface area by BET method ofgenerally from 25 to 80 m² /g, preferably from 35 to 70 m² /g. If thespecific surface area is less than 25 m² /g, the noise can be undulyaccentuated. If, on the contrary, if it is more than 80 m² /g, thesurface property would be unfavorably poor. The crystallite size of theferromagnetic powder of the upper magnetic layer of the medium of thepresent invention is generally from 100 to 450 Å, preferably from 100 to350 Å. Saturation magnetization σs of the iron oxide magnetic powder isgenerally 50 emu/g or more, preferably 70 emu/g or more; and that of theferromagnetic metal powder for use in the present invention ispreferably 100 emu/g or more, more preferably from 110 emu/g to 170emu/g. The coercive force Hc of the ferromagnetic metal powder ispreferably from 1100 Oe to 2500 Oe, more preferably from 1400 Oe to 2000Oe. The acicular ratio of the ferromagnetic powder for use in thepresent invention is preferably 18 or less, more preferably 12 or less.

The ferromagnetic powder is desired to have a r1500 of being generally1.5 or less, more preferably 1.0 or less. The r1500 indicates the amountof the remanent magnetization (%) without being reversed, when themagnetic recording medium has been subjected to saturated magnetizationand then a magnetic field of 1500 Oe of the reversed direction isapplied thereto.

The water content of the ferromagnetic powder is desired to fall withinthe range of from 0.01 to 2%. Depending upon the kind of the binder tobe used along with the ferromagnetic powder, the water content of thepowder is desired to be optimized. The tap density of γ- iron oxide foruse in the present invention is desired to be 0.5 g/ml or more, morepreferably 0.8 g/ml or more. Where an alloy powder is used as theferromagnetic powder, the tap density thereof is desired to be from 0.2to 0.8 g/ml. If the tap density is more than 0.8 g/ml, oxidation of theferromagnetic powder would progress too extensively during thedensification of the powder so that a sufficient saturated magnetization(σs) could hardly be attained. If the tap density is less than 0.2 g/ml,dispersion of the powder would often be insufficient.

Where γ-iron oxide is used, the ratio of the divalent iron, if any, tothe trivalent iron therein is preferably from 0 to 20%, more preferablyfrom 5 to 10%. The ratio of the cobalt atoms, if any, to the iron atomsis preferably from 0 to 15%, more preferably from 2 to 8%.

The pH value of the ferromagnetic powder is desirably optimized,depending upon its particular combination with the binder. The range ofthe pH value is generally from 4 to 12, preferably from 6 to 10. Theferromagnetic powder may optionally be surface-treated with agents suchas Al, Si, P and/or oxides of them. The amount of the agent for thesurface-treatment is from 0.1 to 10% based on the amount offerromagnetic powder. The surface-treatment of the powder is preferred,since the adsorption amount of a lubricant such as fatty acids to thepowder otherwise may be 100 mg/m² or less. The ferromagnetic powderoften contains soluble inorganic ions such as Na, Ca, Fe, Ni and/or Srions. However, such ions have no influence on the characteristics of thepowder, provided that the content of them therein is 500 ppm or less.

The ferromagnetic powder for use in the present invention is desired tobe less porous, and the porosity in the ferromagnetic powder is desiredto be 20% by volume or less, more preferably 5% by volume or less. Theshape of the ferromagnetic powder is not specifically defined and may beany of acicular, granular, ellipsoidal or tabular, provided that thepowder satisfies the characteristics of the previously mentionedparticle size. Where an acicular ferromagnetic powder is used, theacicular ratio thereof is desired to be 12 or less. In order that theferromagnetic powder for use in the present invention may have SFD being0.6 or less, the distribution of Hc of the powder is needed to benarrow. To accomplish this, various methods may be employable. Forinstance, the particle size distribution of goethite is narrowed,sintering of γ-hematite is prevented, and the coating speed of cobalt toiron oxide for cobalt modification is retarded.

In the present invention, also usable, as tabular hexagonal ferrites,are various substituted ferrites such as barium ferrite, strontiumferrite, lead ferrite and calcium ferrite, as well as Co-substitutedferrites and hexagonal Co-ferrite powder. Specifically mentioned aremagnetoplumbite type barium ferrite and strontium ferrite, as well aspartially spinel phase-containing magneto-plumbite type barium ferriteand strontium ferrite. Of them, especially preferred are substitutedferrites of barium ferrite and strontium ferrite. Various elements suchas Co--Ti, Co--Ti--Zr, Co--Ti--Zn, Ni--Ti--Zn or Ir--Zn may be added tothe preceding hexagonal ferrites so as to control their coercive force.

A hexagonal ferrite generally comprises 6-angular tabular particles, andthe particle size diameter means the width of the major plane of each6-angular tabular particle and is measured with an electronicmicroscope. The particles to be used in the present invention arepreferably defined to those having a particle size of from 0.01 to 0.2μm, especially preferably from 0.03 to 0.1 μm. The mean thickness(tabular thickness) of the fine particles is preferably from about 0.001to 0.2 μm, especially preferably from about 0.003 to 0.05 μm. The aspectratio (tabular diameter/tabular thickness) is generally from 1 to 10,preferably from 3 to 7. The fine hexagonal ferrite powder has a specificsurface area by BET method (S_(BET)) of preferably from 25 to 70 m² /g,and a specific gravity of preferably from 4 to 6.

The binder to be used in the present invention for the upper magneticlayer and lower nonmagnetic layer may be selected independently from anyknown thermoplastic resin, thermosetting resin or reactive resin or amixture of them. The thermoplastic resin for use in the presentinvention is one having a glass transition temperature of generally from-100° to 150° C., a number average molecular weight or generally from1,000 to 200,000 preferably from 10,000 to 100,000, and a polymerizationdegree of approximately from 50 to 1000. Examples thereof includepolymers or copolymers comprising constitutive units of vinyl chloride,vinyl acetate, vinyl alcohols, maleic acid, acrylic acid, acrylates,vinylidene chloride, acrylonitrile, methacrylic acid, methacrylates,styrene, butadiene, ethylene, vinyl butyrals, vinyl acetals and/or vinylethers, as well as polyurethane resins and various rubber resins.Example of thermosetting resins and reactive resins for use in thepresent invention include phenolic resins, epoxy resins, hardening typepolyurethane resins, urea resins, melamine resins, alkyd resins, acrylicreactive resins, formaldehyde resins, silicone resins, epoxypolyamideresins, mixtures of polyester resins and isocyanate prepolymers,mixtures of polyester polyols and polyisocyanates, and mixtures ofpolyurethanes and polyisocyanates.

The resins are described in detail in Plastic Handbook (published byAsakura Shoten). Known electronic ray-hardening resins may beincorporated into the lower layer or upper layer of the medium of thepresent invention. Examples thereof including a method of preparationare described in detail in JP-A-62-256219. The above-mentioned resinsmay be used singly or in combination of them in the present invention.Of these, preferred are a combination of a polyisocyanate and at leastone selected from the group consisting of vinyl chloride resins, vinylchloride-vinyl acetate resins, vinyl chloride-vinyl acetate-vinylalcohol resins and vinyl chloride-vinyl acetate-maleic anhydridecopolymers, and a polyurethane resin, and combination thereof.

Examples of polyurethane resins for use in the present invention includeknown polyester polyurethanes, polyether polyurethanes, polyetherpolyester polyurethanes, polycarbonate polyurethanes, polyesterpolycarbonate polyurethanes and polycaprolactone polyurethanes. In orderto attain even further improved dispersibility and durability, it ispreferred, optionally, to introduce into the binders exemplified hereinat least one or more polar groups selected from COOM, SO₃ M, OSO₃ M,P═O(OM)₂, O--P═O(OM)₂ (wherein M is a hydrogen atom or an alkali metal),OH, NR₂, N+R₃ (wherein R is a hydrocarbon residue), an epoxy group, SHand CN by copolymerization or addition reaction. The amount of suchpolar groups in the binder, if any, is generally from 1×10⁻¹ to 1×10⁻⁸mol/g, preferably from 1×10⁻² to 1×10⁻⁶ mol/g.

Specific examples of the binders for use in the present inventioninclude VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL,XYSG, PKHH, PKHJ, PKHC, PKFE manufactured by Union Carbide Co., Ltd.;MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM, MPR-TAOmanufactured by Nisshin Chemical Industry Co., Ltd.; 1000W, DX80, DX81,DX82, DX83, 100FD manufactured by Electro Chemical Industry Co., Ltd.;MR105, MR110, MR100, 400X100A manufactured by Nippon Zeon Co., Ltd.;NIPPOLLAN N2301, N2302, N2304 manufactured by Nippon Polyurethane Co.,Ltd.; PANDEX T-5105, T-R3080, T-5201, VERNOCK D-400 D-210-80, CRISVON6109, 7209 manufactured by Dai-Nippon Ink and Chemical Inc.; BYLONUR8200, UR8300, UR8600, UR5500, UR4300, RV530, RV280 manufactured byToyobo Co., Ltd.; DAIPHERAMINE 4020, 5020, 5100, 5300, 9020, 9022, 7020manufactured by Dainichi Seika Co., Ltd.; MX5004 manufactured byMitsubishi Chemical corporation; SUNPRENE SP-150 manufactured by SanyoChemical Industries Co., Ltd.; and SALAN F310, F210 manufactured byAsahi Chemical Industry Co., Ltd.

The proportion of the binder in the upper magnetic layer of the mediumof the present invention is generally from 5 to 50% by weight,preferably from 10 to 30% by weight, based on the ferromagnetic powdertherein. Where vinyl chloride resins are used, the proportion is from 5to 30% by weight; where polyurethane resins are used, it is from 2 to20% by weight; and where polyisocyanates are used, it is from 2 to 20%by weight. A combination thereof is preferably used.

The proportion of the non-magnetic powders in the lower non-magneticlayer of the medium of the present invention is generally from 20 to 0.1by weight, preferably from 15 to 0.5 by weight and more preferably from10 to 1 by weight, and generally from 10 to 0.1 by volume, preferablyfrom 8 to 0.5 by volume and more preferably from 5 to 1 by volume, basedon the amount of binder in the lower layer.

When polyurethane resins are used in the present invention, theypreferably have a glass transition temperature of from -50° to 100° C.,a breaking point elongation of from 100 to 2000%, a breaking pointelongation of from 100 to 2000%, a breaking point stress of from 0.05 to10 kg/cm², and a yield value of from 0.05 to 10 kg/cm².

The magnetic recording medium of the present invention basicallycomprises two layers, but may comprise three or more layers. Asconstitution of the medium comprising three or more layers, the uppermagnetic layer is composed of two or more plural magnetic layers. Inthis case, common knowledge of plural magnetic layer designs can applyto the relationship between the upper-most layer and the other lowermagnetic layers. For instance, the uppermost magnetic layer has a highercoercive force than the other lower magnetic layers, and the formercontains a ferromagnetic powder having a smaller mean longer axis lengthand a smaller crystallite size than the latter. As the case may be, thelower non-magnetic layer of the medium of the present invention may becomposed of plural non-magnetic layers. In principle, the medium iscomposed of the upper magnetic layer unit and the lower non-magneticlayer unit.

It is of course possible to vary the amount of the binder, the amountsof vinyl chloride resins, polyurethane resins, polyisocyanates and otherresins in the binder, the molecular weight of each resin of constitutingthe magnetic layer, the amount of the polar groups, if any, in thebinder, and the physical characteristics of the resins, in accordancewith the needs and circumstances as between the particular lower layerand the particular upper magnetic layer.

Examples of polyisocyanates for use in the present invention includeisocyanates such as tolylene diisocyanate, 4,4'-diphenylmethanediisocyanate, hexamethylene diisocyanate, xylylene diisocyanate,naphthylelne-1,5-di-isocyanate, o-toluidine isocyanate, isophoronediisocyanate and triphenylmethane triisocyanate; products of theseisocyanates and polyalcohols; and polyisocyanates to be formed bycondensation of these isocyanates. As commercial products of theseisocyanates, there are CORONATE L, CORONATE HL, CORONATE 2030, CORONATE2031, MILLIONATE MR, MILLIONATE MTL manufactured by Nippon PolyurethaneCo., Ltd.; TAKENATE D-102, TAKENATE D-110N, TAKENATE D-200, TAKENATED-202 manufactured by Takeda Chemicals Industries Co., Ltd.; andDESMODURL, DESMODUR IL, DESMODURN, DESMODUR HL manufactured by SumitomoBayer Co., Ltd. These polyisocyanates can be incorporated into the lowernon-magnetic layer and the upper magnetic layer, singly or incombination of two or more of them on the basis of the relativedifference in the hardening reactivity between them.

The carbon black to be optionally used in the upper magnetic layer ofthe medium of the present invention may be any of furnace black forrubbers, thermal black for rubbers, carbon black for coloration andacetylene black. It is preferred that the carbon black has a specificsurface area of from 5 to 500 m² /g, a DBP oil absorption amount of from10 to 400 ml/100 g, a particle size of from 5 mμ to 300 mμ, a pH valueof from 2 to 10, a water content of from 0.1 to 10% and a tap density offrom 0.1 to 1 g/ml. Specific examples of the carbon black, includeBLACKPEARLS 2000, 1300, 1000, 900, 800, 700, VULCAN XC-72 manufacturedby Cabot Co., Ltd.; #80, #60, #55, #50, #35 manufactured by Asahi CarbonCo., Ltd.; #2400B, #2300, #900, #1000, #30, #40, #10B manufactured byMitsubishi Chemical Corporation; and CONDUCTEX SC, RAVEN 150, 50, 40, 15manufactured by Columbia Carbon Co., Ltd. The carbon black for use inthe present invention may be surface-treated with a dispersing agent, ormay be grafted with a resin, or the surface thereof may be partlygraphatized. The carbon black may previously be dispersed in a binderprior to addition thereof to the magnetic coating solution. These carbonblacks may be used singly or in combination of two or more thereof. Theamount of the carbon black in the magnetic layer is preferably from 0.1to 30% based on the amount of the ferromagnetic powder therein. Thecarbon black acts for prevention of static charges, reduction offriction factor, impartation of light-shielding property and elevationof film strength. The action differs between the kinds of the carbonblack to be used. Therefore, it is of course possible to differentlyincorporate various carbon blacks into the lower layer and the upperlayer constituting the medium of the present invention on the basis ofthe kind and amount of them and the way of combination of them and alsoon the basis of other various characteristics of them, such as theparticle size, the oil absorption amount, the electroconductivity andthe pH value thereof, in accordance with the object desired. For thecarbon blacks used in the upper layer of the medium of the presentinvention, for example, the disclosure of Carbon Black Handbook(published by Carbon Black Association) may be referred to.

The upper magnetic layer of the medium of the present invention maycontain an abrasive. As such an abrasive, usable are various knownmaterials having a Mohs' hardness of 6 or more, singly or in combinationthereof. They include, for example, α-alumina having an alphatizationdegree of 90% or more, β-alumina, silicon carbide, chromium oxide,cerium oxide, α-iron oxide, corundum, synthetic diamond, siliconnitride, silicon carbide, titanium carbide, titanium oxide, silicondioxide, and boron nitride. A composite of these abrasives (as preparedby surface-treating one abrasive with another abrasive) may also beused. The abrasives can contain any other compounds or elements otherthan the essential component, which may also be used in the presentinvention to attain the same effect, provided that the content of theessential component therein is 90% or more. The abrasives are desired tohave a particle size of from 0.01 to 2 μm. If desired, a combination ofplural abrasives each having a different particle size may be employedin the present invention. As the case may be, a single abrasive having abroad particle size distribution may also be used to attain the sameeffect. The abrasives are desired to have a tap density of from 0.3 to 2g/ml, a water content of from 0.1 to 5%, a pH value of from 2 to 11, anda specific surface area of from 1 to 30 m² /g. The shape of theabrasives for use in the present invention may be any of acicular,spherical or cubic one. Preferred are those having angular corners asthe shape thereof, as such have high abrasiveness.

Specific examples of the abrasive for use in the present inventioninclude AKP-20, AKP-30, AKP-50, HIT-50 manufactured by Sumitomo ChemicalCo., Ltd.; G5, G7, S-1 manufactured by Nippon Chemical Industry Co.,Ltd.; and TF-100, TF-140, 100ED, 140ED manufactured by Toda Kogyo Co.,Ltd. It is, of course, possible to differently incorporate variousabrasives of different kinds and different amounts in differentcombinations into the lower layer and the upper layer constituting themedium of the present invention, in accordance with the object desired.The abrasive may previously be dispersed in a binder prior to itsaddition into the magnetic coating solution. The amount of the abrasiveparticles present in the surface of the upper magnetic layer and theedges of the medium of the present invention is preferably 5particles/100 μm² or more.

To the medium of the present invention may be added various additives,such as those having a lubricating effect, antistatic effect, dispersingeffect or plasticizing effect. Examples of additives used in the presentinvention include molybdenum disulfide, tungsten disulfide, graphite,boron nitride, graphite fluoride, silicone oil, polar group-containingsilicones, fatty acid-modified silicones, fluorine-containing silicones,fluorine-containing alcohols, fluorine-containing esters, polyolefins,polyglycols, alkylphosphates and their alkali metal salts, alkylsulfatesand their alkali metal salts, polyphenyl ethers, fluorine-containingalkylsulfates and their alkali metal salts, monobasic fatty acids havingfrom 10 to 24 carbon atoms (optionally unsaturated or branched) andtheir metal salts (with Li, Na, K, Cu), mono-, di-, tri-, tetra-, hepta-or hexa-alcohols having from 12 to 22 carbon atoms (optionallyunsaturated or branched), alkoxyalcohols having from 12 to 22 carbonatoms, fatty acid monoesters, fatty acid diesters or fatty acidtriesters composed of monobasic fatty acids having from 10 to 24 carbonatoms (optionally unsaturated or branched) and anyone of mono-, di-,tri-, tetra-, hepta- or hexa-alcohols having from 2 to 12 carbon atoms(optionally unsaturated or branched), fatty acid esters of monoalkylethers of alkylene oxide polymers, fatty acid amides having from 8 to 22carbon atoms, and aliphatic amines having from 8 to 22 carbon atoms.Specific examples thereof include lauric acid, myristic acid, palmiticacid, stearic acid, behenic acid, butyl stearate, oleic acid, linolicacid, linolenic acid, elaidic acid, octyl stearate, amyl stearate,isooctyl stearate, octyl myristate, butoxyethyl stearate,anhydrosorbitan mono-stearate, anhydrosorbitan distearate,anhydrosorbitan tri-stearate, oleyl alcohol, and lauryl alcohol arementioned.

In addition, also usable in the present invention are nonionicsurfactants such as alkylene oxides, glycerins, glycidols andalkylphenol-ethyleneoxide adducts; cationic surfactants such as cyclicamines, ester amides, quaternary ammonium salts, hydantoin derivatives,heterocyclic compounds, phosphoniums and sulfoniums; anionic surfactantsof containing acid groups such as carboxylic acid group, sulfonic acidgroup, phosphoric acid group, sulfate group and phosphate group; andamphoteric surfactants such as amino acids, aminosulfonic acids,sulfuric acid or phosphoric acid esters of aminoalcohols andalkylbetains. These surfactants are described in detail in surfactantHandbook (published by Sangyo Tosho KK). The lubricant and antistaticagent for use in the present invention need not necessarily be 100%pure. As the case may be, they may contain various impurities, inaddition to the essential component, such as isomers, nonreactedmaterials, side products, decomposates and oxides. The content of theimpurities in the agent is desired to be preferably 30% or less, morepreferably 10% or less.

The lubricant and surfactant may differently be incorporated into thelower non-magnetic layer and the upper magnetic layer of constitutingthe medium of the present invention, with respect to the kind and amountof them, in accordance with the particular necessity. For instance,these additives may be incorporated into the necessary layers in such away that different fatty acids each having a different melting point areincorporated differently in the lower non-magnetic layer and the uppermagnetic layer so as to prevent bleeding of the coated layers, thatdifferent esters each having a different boiling point or havingdifferent polarity are incorporated therein also so as to preventbleeding of the coated layers, that the amount of the surfactant to bein the coating solution is controlled to elevate the coating stability,and that the amount of the lubricant is made larger in the lowernon-magnetic layer so as to elevate the lubricating effect. Needless tosay, the illustrated embodiments herein are not to be consideredlimitative.

All or a part of the additives to be used in the layers constituting themedium of the present invention may be added to the coating solution atany step of preparing the solutions. For instance, they may be blendedwith a ferromagnetic powder prior to kneading; they may be added to amixture of a ferromagnetic powder, a binder and a solvent during thestep of kneading them; they may be added during dispersion or afterdispersion; or they may be added immediately before coating. Examples ofcommercial products of the lubricant for use in the present inventioninclude NAA-102, NAA-415, NAA-312, NAA-160, NAA-180, NAA-174, NAA-175,NAA-222, NAA-34, NAA-35, NAA-171, NAA-122, NAA-142, NAA-160, NAA-173K,castor oil-hardened fatty acids, NAA-42, NAA-44, CATION SA, CATION MA,CATION AB, CATION BB, NYMEEN L-201, NYMEEN L-202, NYMEEN S-202, NONIONE-208, NONION P-208, NONION S-207, NONION K-204, NONION NS-202, NONIONNS-210, NONION HS-206, NONION L-2, NONION S-2, NONION S-4, NONION 0-2,NONION LP-20R, NONION PP-40R, NONION SP-60R, NONION OP-80R, NONIONOP-85R, NONION LT-221, NONION ST-221, NONION TO-221, MONOGURI MG, NONIONDS-60, ANON BF, ANON LG, butyl stearate, butyl laurate, erucic acidmanufactured by Nippon Oils & Fats Co., Ltd.; oleic acid by KantoChemical Co., Ltd.; FAL-205, FAL-123 by Takemoto Yushi Co., Ltd.;Enujerubu LO, Enujerubu IPM, Sansosyzer E4030 by Shin-Nippon Rika Co.,Ltd.; TA-3, KF-96, KF-96L, KF-96H, KF410, KF420, KF965, KF54, KF50,KF56, KF-907, KF-851, X-22-818, X-22-822, KF-905, KF-700, KF-393,KF-857, KF-860, KF-865, X-22-980, KF-101, KF-102, KF-103, X-22-3710,X-22-3715, KF-910, KF-3935 manufactured by Shin-Etsu Chemical Co., Ltd.;ARMIDE P, ARMIDE C, ARMOSLIP CP manufactured by Lion Armer Co., Ltd.;DUOMIN TDO manufactured by Lion Fat & Oil Co., Ltd.; BA-41G manufacturedby Nisshin Oil Mills Co., Ltd.; and PROFAN 2012E, NEWPOLE PE61, IONETMS-400, IONET MO-200, IONET DL-200, IONET DS-300, IONET DS-1000, IONETDO-200 manufactured by Sanyo Chemical Co., Ltd.

The coating solutions for the layers of the medium of the presentinvention can contain organic solvents in any desired proportion. Suchorganic solvents include, for example, ketones such as acetone, methylethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone,isophorone, tetrahydrofuran; alcohols such as methanol, ethanol,propanol, butanol, isobutyl alcohol, isopropyl alcohol,methylcyclohexanol; esters such as methyl acetate, butyl acetate,isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate;glycol ethers such as glycol dimethylether, glycol monoethylether,dioxane; aromatic hydrocarbons such as benzene, toluene, xylene, cresol,chlorobenzene; chlorinated hydrocarbons such as methylene chloride,ethylene chloride, carbon tetrachloride, chloroform, ethylenechlorohydrin, dichlorobenzene; and N,N-dimethylformamide and hexane.These organic solvents are not needed to be 100% pure ones but maycontain various impurities, in addition to the main component, such asisomers, non-reacted materials, side products, decomposates, oxides andwater. The content of the impurities therein is desired to be 30% byweight or less, more preferably 10% by weight or less. The kind andamount of the organic solvents to be in the upper layer and the lowerlayer of constituting the medium of the present invention may be varied.As examples of the ways of incorporating the organic solvents to thelayers, mentioned are such that a more volatile solvent is used in thelower layer so as to elevate the surface property; a solvent having ahigher surface tension (e.g., cyclohexanone, dioxane) is used in themagnetic layer so as to elevate the coating stability; and a solventhaving a higher dissolution parameter is used in the magnetic layer soas to elevate the filling density. Needless to say, however, theillustrated examples herein are not limitative.

Regarding the thickness constitution of the magnetic recording medium ofthe present invention, the thickness of the non-magnetic support isgenerally from 1 to 100 μm, preferably from 4 to 20 μm, more preferablyfrom 5 to 12 μm; the mean thickness of the lower layer is generally from1 to 5 μm; and the mean thickness of the upper layer is generally from0.05 μm to 1.0 μm, preferably from 0.05 μm to 0.6 μm, more preferablyfrom 0.05 μm to 0.3 μm. The total thickness of both the upper layer andthe lower layer is 1/100 to 2 times of the thickness of the non-magneticsupport. An undercoating layer may be provided between the non-magneticsupport and the lower layer so as to elevate the adhesion therebetween.The thickness of the undercoating layer is generally from 0.01 to 2 μ,preferably from 0.05 to 0.5 μm. A back coating layer maybe provided onthe back surface of the non-magnetic layer opposite to the magneticlayer. The thickness of the back coating layer is generally from 0.1 to2 μm, preferably from 0.3 to 1.0 μm. Such an undercoating layer and aback coating layer may be conventional ones. It is effective toincorporate the powder (C), which is used in the lower layer, also intothe back coating layer so as to better prevent the edge damage of themedium.

As the non-magnetic support of the medium of the present invention,usable are any known films of polyesters such as polyethyleneterephthalate or polyethylene napthalate, as well as polyolefins,cellulose triacetate, polycarbonates, polyamides, polyimides,polyamidoimides, polsulfones, aramides or aromatic polyamides. Thesupport may previously be subjected to corona discharging treatment,plasma treatment, easy adhesion treatment, heat treatment, dust removingtreatment, etc. In order to attain the object of the present invention,the non-magnetic support is one having a center line mean surfaceroughness of generally 0.03 μm or less, preferably 0.02 μm or less, morepreferably 0.01 μm or less. In addition, it is desired that the supportnot only has such a small center line mean surface roughness but alsodoes not have large projections of 1 μm or more. The roughness profileof the surface of the support may be freely controlled in accordancewith the size and the amount of the filler to be added to the support,if desired. Examples of the filler include oxides and carbonates of Ca,Si and Ti, as well as organic fine powders of acrylic substances. TheF-5 value (i.e., the load at the 5% elongation) of the non-magneticsupport in the tape running direction is preferably from 5 to 50 kg/mm²,and that in the tape-width direction is preferably from 3 to 30 kg/mm² .In general, the F-5 value in the tape-running (lengthwise) direction ishigher than that in the tape width direction. However, when the strengthof the tape in the tape-width direction is desired to be high, thisprescription shall not apply.

The thermal shrinkage (percentage) of the non-magnetic support in boththe tape running direction and the tape width direction at 100° C. for30 minutes is preferably 3% or less, more preferably 1.5% or less; andthe same at 80° C. for 30 minutes is preferably 1% or less, morepreferably 0.5% or less. The strength of the support at the breakingpoint is preferably from 5 to 100 kg/mm² in the both directions; and themodulus of elasticity of the same is preferably from 100 to 2000 kg/mm²in the both directions.

The process of producing the magnetic coating solution for the magneticrecording medium of the present invention comprises at least a kneadingstep, a dispersing step and optionally a mixing step to be providedbefore and after the preceding steps. The respective steps each may becomposed of two or more stages. In producing the composition, all theraw materials of the ferromagnetic powder, binder, carbon black,abrasive, antistatic agent, lubricant and solvent may be added to thereactor initially at the start of the process or later during the courseof the process. The individual raw materials may divided into pluraldivisions to be added in two or more steps of the process. For instance,polyurethane is divided into plural divisions and added in the kneadingstep and dispersing step and also in the mixing step for adjustment ofthe viscosity after dispersion.

For attaining the object of the present invention, any knownconventional technology may of course be employed as a part of theprocess of producing the magnetic recording medium of the presentinvention. For instance, in the kneading step, a kneading machine havinga strong kneading power, such as a continuous kneader or a pressurekneader, may be employed so as to obtain the magnetic recording mediumwith high Br of the present invention. Where such a continuous kneaderor a pressure kneader is employed, a ferromagnetic powder is kneadedwith all or a part (preferably 30% by weight or more) of a binder. Forinstance, 100 parts by weight of a ferromagnetic powder is blended withfrom 15 to 500 parts by weight of a binder. The details of the kneadingtechnology are described in JP-A-1-106338 and JP-A-64-79274.

As examples of the apparatus and process for producing the magneticrecording medium of the present invention, which is composed of plurallayers coated on a support, the following are referred to.

1. A lower layer is first coated on a support with a gravure coating,roll coating, blade coating or extrusion coating device, which isgenerally employed for coating a magnetic coating composition, and whilethe coating lower layer is still wet, an upper layer is coated thereoverwith a support-pressing extrusion coating device as illustrated inJP-B-1-46186 and JP-A-60-238179 and JP-A-2-265672.

2. A lower layer and an upper layer are almost simultaneously coated ona support with one coating head having therewith two coating solutionpassing slits, as illustrated in JP-A-63-88080, JP-A-2-17921, andJP-A-2-265672.

3. A lower layer and an upper layer are almost simultaneously coated ona support with a back-up roll-equipped extrusion coating device asillustrated in JP-A-2-174965.

In order to prevent lowering of the electromagnetic characteristics ofthe magnetic recording medium due to aggregation of the ferromagneticpowder coated, it is desired to apply a shearing force to the coatingsolution in the inside of the coating head by the method described inJP-A-62-95174 and JP-A-1-236968. The viscosity of the coating solutionis desired to satisfy the numerical range as disclosed in JP-A-3-8471.

In order to obtain the medium of the present invention, strongorientation is necessary. For this purpose, a solenoid of 1000 G (gauss)or more and a cobalt magnet of 2000 G or more are used in combination.In order that the orientation of the dried medium may be the highest, itis preferred that the medium is previously suitably dried prior toorientation. Where the medium of the present invention is a disc,randomizing orientation is typically necessary.

As calendering rolls to be used in producing the medium of the presentinvention, usable are heat-resistant plastic rolls made of epoxy,polyimide, polyamide or polyimidoamide resins. Calendering may also beeffected between two metal rolls. The calendering temperature ispreferably 70° C. or higher, more preferably 80° C. or higher. Thelinear pressure for Calendering is preferably 200 kg/cm or more, morepreferably 300 kg/cm or more.

The coefficient of friction against SUS420J, of the upper layer of themagnetic recording medium and that of the opposite surface of the sameare preferably 0.5 or less, more preferably 0.3 or less. The surfaceintrinsic resistance of the magnetic layer is preferably from 1×10⁴ to1×10¹¹ Ω/sq. The modulus of elasticity of the magnetic layer at 0.5%elongation is preferably from 100 to 2000 kg/mm² both in the runningdirection and the width direction; the strength of the layer at thebreaking point is preferably from 1 to 30 kg/cm² ; the modulus ofelasticity of the magnetic recording medium of the invention ispreferably from 100 to 500 kg/mm² in both the running direction and thewidth direction; the residual elongation of the same is preferably 0.5%or less; the thermal shrinkage (percentage) of the same at everytemperature of 100° C. or lower is preferably 1% or less, morepreferably 0.5% or less, most preferably 0.1% or less.

The content of the solvent remaining in the upper layer of the medium ofthe present invention is preferably 100 mg/m² or less, more preferably10 mg/m² or less; and the amount of the remaining solvent in the upperlayer is desired to be smaller than that in the lower layer.

The voltage of the upper layer and that of the lower layer each arepreferably 30% by volume or less, more preferably 20% by volume or less.The voidage of the lower layer is preferably smaller than that of theupper magnetic layer. As the case may be, the voidage of the lower layermay often be higher than that of the upper layer, depending upon theobject. For instance, for a magnetic recording medium for recordingdata, for which repeated use is considered important, the reverserelationship is often preferred.

Regarding the magnetic characteristics of the magnetic recording mediumof the present invention as measured in a magnetic field of 5 KOe, thesquareness ratio in the tape running direction is generally 0.70 ormore, preferably 0.80 or more, more preferably 0.90 or more. Thesquareness ratio in the two directions perpendicular to the tape runningdirection is desired to be 80% or less of that in the tape runningdirection. The SFD of the magnetic layer is desired to be 0.6 or less.

The center line mean surface roughness (Ra) of the magnetic layer isdesired to be from 2 nm to 20 nm, and the value is to be suitablydefined in accordance with the object. For improving the electromagneticcharacteristics, Ra is desired to be smaller. However, for improving therunning durability, Ra is desired to be larger. RMS surface roughness(R_(RMS)) of the magnetic layer as obtained by STM evaluation is desiredto fall within the range of from 3 nm to 16 nm.

The present invention will be explained in more detail by way of thefollowing examples, which, however, are not intended to restrict thescope of the present invention. Unless otherwise specifically indicated,all "parts" are "parts by weight".

EXAMPLE 1

A coating solution for the lower non-magnetic layer and a coatingsolution for the upper magnetic layer were prepared, each having thecomposition described below.

    ______________________________________                                        Coating Solution for Lower Non-Magnetic Layer:                                Inorganic powder (powder A) TiO.sub.2                                                                   75     parts                                        mean particle size: 0.035 μm                                               type of crystal: rutile                                                       TiO.sub.2 content: 90% or more                                                specific surface area by BET method: 40 m.sup.2 /g                            DBP oil absorption amount: 27 to 38 g/100 g                                   pH 7                                                                          surface-treating agent: Al.sub.2 O.sub.3                                      Carbon Black (powder B)   15     parts                                        mean particle size: 16 mμ                                                  DBP oil absorption amount: 80 ml/100 g                                        pH 8.0                                                                        specific surface area by BET method: 250 m.sup.2 /g                           volatile content: 1.5%                                                        Coarse particles (powder C)                                                                             10     parts                                        α-Al.sub.2 O.sub.3 (HIT-55, by Sumitomo Chemical Co., Ltd.)             mean particle size: 0.2 μm                                                 specific surface area by BET method: 9 m.sup.2 /g                             Vinyl chloride-vinyl acetate-vinyl                                                                      12     parts                                        alcohol copolymer                                                             containing 5 × 10.sup.-6 eq/g of polar group --N(CH.sub.3).sub.3.sup    .+ Cl.sup.-                                                                   composition ratio: 86/13/1                                                    polymerization degree: 400                                                    Polyester polyurethane resin                                                                            5      parts                                        neopentyl glycol/caprolactone polyol/                                         MDI = 0.9/2.6/1                                                               containing 1 × 10.sup.-4 eq/g of --SO.sub.3 Na group                    Butyl Stearate            1      part                                         Stearic Acid              1      part                                         Methyl Ethyl Ketone       200    parts                                        Coating Solution for Upper Magnetic Layer:                                    Fine Powder of Ferromagnetic Metal                                                                      100    parts                                        with composition of Fe/Zn/Ni = 92/4/4                                         Hc: 1600 Oe                                                                   specific surface area by BET method: 60 m.sup.2 /g                            crystallite size: 195 Å                                                   mean length in the long axis: 0.20 μm                                      acicular ratio: 10                                                            saturated magnetization (σ.sub.s): 130 emu/g                            Vinyl Chloride Copolymer  12     parts                                        containing 1 × 10.sup.-4 eq/g of --SO.sub.3 Na group                    polymerization degree: 300                                                    Polyester Polyurethane Resin                                                                            3      parts                                        neopentyl glycol/caprolactone polyol/                                         MDI = 0.9/2.6/1                                                               containing 1 × 10.sup.-4 eq/g of --SO.sub.3 Na group                    α-alumina (mean particle size 0.3 μm)                                                          2      parts                                        Carbon Black (mean particle size 0.10 μm)                                                            0.5    part                                         Butyl Stearate            1      part                                         Stearic Acid              2      parts                                        Methyl Ethyl Ketone       200    parts                                        ______________________________________                                    

Each of the preceding two coating solutions was kneaded in a continuouskneader and then dispersed with a sand mill. Polyisocyanate was added tothe both dispersions thus obtained, in an amount of one part to thedispersion for the coating solution of the lower non-magnetic layer andin an amount of 3 parts to that for the coating solution of the uppermagnetic layer. Further, 40 parts of butyl acetate was added to each ofthem. These dispersions were then filtered each through a filter havinga mean pore size of 1 μm. Thus, a coating solution for the lowernon-magnetic layer and a coating solution for the upper magnetic layerwere prepared.

The coating solution for the lower non-magnetic layer was coated on apolyethylene terephthalate support having a thickness of 7 μm and acenter line mean surface roughness of 0.01 μm, in a dry thickness of 2μm and, immediately after the coating, the coating solution for theupper magnetic layer was coated thereover in a dry thickness of 0.5 μm,by simultaneous multi-coating method. While the both layers were stillwet, the coated support was oriented with a cobalt magnet having amagnetic power of 3000 G and a solenoid having a magnetic powder of 1500G. The oriented coated support was then dried and calendered with a7-stage calendering device composed of only metal rolls at a temperatureof 90° C., and this was slit into a width of 8 mm to prepare a 8 mmvideo tape of Example 1-1.

In the same manner as above, samples of Examples 1-2 to 1-8 andcomparative samples of Comparative Examples 1-1 to 1-9 were prepared,except that the factors as indicated in Tables 1 to 3 below were used.These samples were tested by the methods mentioned below, and theresults obtained are shown in Tables 1 to 3.

1. Volume ratio of powders A, B and C:

The volume of each powder was calculated by dividing the respectiveweight ratio of each of the components constituting the lowernon-magnetic layer by the true specific gravity of each powder, and thevolume ratio of the respective powder components was obtained on thebasis of the sum of the powder volume being 100%.

2. 7 MHz output power:

Using a 8 mm video deck of "FUJIX8" (manufactured by Fuji Photo FilmCo., Ltd.), a 7 MHz signal was recorded on each sample and the recordedsignal was reproduced, whereupon the 7 MHz signal reproducing power wasmeasured with an oscilloscope. As a control, used was a 8 mm tape SAGP6-120 (manufactured by Fuji Photo Film Co., Ltd.).

3. C/N ratio:

Using a 8 mm video deck of FUJIX8 Model (manufactured by Fuji Photo FilmCo., Ltd.), a 7 MHz signal was recorded on each sample and the recordedsignal was reproduced, whereupon the noise as generated at 6 MHz wasmeasured with a spectroanalyzer. The ratio of the reproduced signal tothe noise was determined.

4. Still durability:

Using a 8 mm video deck of "FUJIXS" (manufactured by Fuji Photo FilmCo., Ltd.), a 7 MHz signal was recorded on each sample in an atmosphereof 5° C. and 80% RH, and the recorded signal was reproduced by stillmode, whereupon the time spent for lowering the output power by 6 dB ormore was measured to be the still life. The measurement was finished in60 minutes.

5. Running durability:

Each sample was run through ten "FUJIX8" 8 mm video decks (allmanufactured by Fuji Photo Film Co., Ltd.) 100 times for each, in anatmosphere of 23° C. and 70% RH. During the running test, decrease ofthe output was measured. After the test, the parts of each deck werechecked as to whether or not and how they got soiled and the edgedamage, if any, of the tested sample was also checked.

In Table 3 below, "G" indicates that the decrease of the output powerwas 3 dB or less and the soiling of the parts of each deck was notobservable with the naked eye; "M" indicates that the depression of theoutput power was 3 dB or less but the parts of each deck were observedto get soiled with the naked eye; and "B" indicates that the depressionof the output power was more than 3 dB and the parts of each deck weresoiled extensively.

                                      TABLE 1                                     __________________________________________________________________________                                Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                                         Ex. 1-1                                                                           Ex. 1-2                                                                           Ex. 1-3                                                                           Ex. 1-4                                                                           Ex. 1-5                                                                           Ex. 1-1                                                                           Ex. 1-2                                                                           Ex. 1-3                                                                           Ex. 1-4                               __________________________________________________________________________    Powder A                                                                      kind    TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                             specific gravity                                                                      4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2                                   mean particle size                                                                    0.035                                                                             0.021                                                                             0.075                                                                             0.035                                                                             0.035                                                                             0.1 0.035                                                                             0.035                                                                             0.009                                 (μm)                                                                       content, as weight                                                                    75  75  75  75  75  75  75  75  75                                    ratio                                                                         volume ratio (%)                                                                      62.8                                                                              62.8                                                                              62.8                                                                              62.8                                                                              62.8                                                                              62.8                                                                              62.8                                                                              62.8                                                                              62.8                                  Powder B                                                                      mean particle size                                                                    0.016                                                                             0.016                                                                             0.016                                                                             0.033                                                                             0.016                                                                             0.016                                                                             0.045                                                                             0.016                                                                             0.016                                 (μm)                                                                       content, as weight                                                                    15  15  15  15  15  15  15  15  15                                    ratio                                                                         volume ratio (%)                                                                      28.4                                                                              28.4                                                                              28.4                                                                              28.4                                                                              28.4                                                                              28.4                                                                              28.4                                                                              28.4                                                                              28.4                                  Powder C                                                                      kind    αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3               specific gravity                                                                      4   4   4   4   4   4   4   4   4                                     mean particle size                                                                    0.2 0.2 0.2 0.2 0.8 0.2 0.2 1   1                                     (μm)                                                                       shape   cubic                                                                             cubic                                                                             cubic                                                                             cubic                                                                             cubic                                                                             cubic                                                                             cubic                                                                             cubic                                                                             cubic                                 content, as weight                                                                    10  10  10  10  10  10  10  10  10                                    ratio                                                                         volume ratio (%)                                                                      8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8                                   __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________            Comp.                                                                             Comp.       Comp.                                                                             Comp.   Comp.                                             Ex. 1-5                                                                           Ex. 1-6                                                                           Ex. 1-6                                                                           Ex. 1-7                                                                           Ex. 1-7                                                                           Ex. 1-8                                                                           Ex. 1-8                                                                           Ex. 1-9                                   __________________________________________________________________________    Powder A                                                                      kind    TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         βFe.sub.2 O.sub.3                                                            βFe.sub.2 O.sub.3                                                            --                                        specific gravity                                                                      4.2 4.2 4.2 4.2 4.2 5.1 5.1 --                                        mean particle size                                                                    0.035                                                                             0.035                                                                             0.035                                                                             0.035                                                                             0.035                                                                             0.05                                                                              0.05                                                                              --                                        (μm)                                                                       content, as weight                                                                    75  75  75  75  75  90  80  0                                         ratio                                                                         volume ratio (%)                                                                      62.8                                                                              62.8                                                                              62.8                                                                              64.0                                                                              62.8                                                                              76.6                                                                              63.4                                                                              0.0                                       Powder B                                                                      mean particle size                                                                    0.009                                                                             0.016                                                                             0.016                                                                             0.016                                                                             0.016                                                                             0.016                                                                             0.016                                                                             0.016                                     (μm)                                                                       content, as weight                                                                    15  15  15  15  15  10  15  50                                        ratio                                                                         volume ratio (%)                                                                      28.4                                                                              28.4                                                                              28.4                                                                              28.9                                                                              28.4                                                                              23.4                                                                              32.6                                                                              72.9                                      Powder C                                                                      kind    αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           TiO.sub.2                                                                         αFe.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           αAl.sub.2 O.sub.3                                                           βFe.sub.2 O.sub.3                                                            αFeOOH                              specific gravity                                                                      4   4   4   5.1 4   4   5.1 5                                         mean particle size                                                                    1   0.06        0.2 0.2 0.4                                           (μm)                                                                       shape   cubic                                                                             cubic                                                                             acicular                                                                          tabular                                                                           cubic                                                                             cubic                                                                             cubic                                                                             acicular                                  size(*)         0.06                                                                              1               0.023                                     aspect ratio    15  14              13                                        content, as weight                                                                    10  10  10  10  10  0   5   50                                        ratio                                                                         volume ratio (%)                                                                      8.8 8.8 8.8 7.0 8.8 0.0 4.0 27.1                                      __________________________________________________________________________     *size (μm): tabular diameter or length in the long axis               

                                      TABLE 3                                     __________________________________________________________________________                                   Comp.                                                                             Comp.                                                                             Comp.                                                                             Comp.                              Ex. 1-1    Ex. 1-2                                                                           Ex. 1-3                                                                           Ex. 1-4                                                                           Ex. 1-5                                                                           Ex. 1-1                                                                           Ex. 1-2                                                                           Ex. 1-3                                                                           Ex. 1-4                                __________________________________________________________________________    Thickness of Upper                                                                       0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5                                Magnetic Layer (μm)                                                        Thickness of Lower Non-                                                                  2   2   2   2   2   2   2   2   2                                  magnetic Layer (μm)                                                        Coating Method (*)                                                                       Multi-                                                                            Multi-                                                                            Multi-                                                                            Multi-                                                                            Multi-                                                                            Multi-                                                                            Multi-                                                                            Multi-                                                                            Multi-                                        coating                                                                           coating                                                                           coating                                                                           coating                                                                           coating                                                                           coating                                                                           coating                                                                           coating                                                                           coating                            Evaluation                                                                    7 MHz Output (dB)                                                                        6.5 7.1 6.3 6   6.3 3.5 2.9 3.1 2.5                                C/N Ratio (dB)                                                                           6.1 6.6 6   6.1 5.8 2.9 2.5 2.6 2.0                                Still Durability                                                                         60 min.                                                                           60 min.                                                                           60 min.                                                                           60 min.                                                                           60 min.                                                                           60 min.                                                                           60 min.                                                                           60 min.                                                                           60 min.                                       or more                                                                           or more                                                                           or more                                                                           or more                                                                           or more                                                                           or more                                                                           or more                                                                           or more                                                                           or more                            Running Durability                                                                       G   G   G   G   G   G   G   G   G                                  __________________________________________________________________________               Comp.                                                                             Comp.         Comp.                                                                             Comp.    Comp.                                          Ex. 1-5                                                                           Ex. 1-6                                                                            Ex. 1-6                                                                           Ex. 1-7                                                                            Ex. 1-7                                                                           Ex. 1-8                                                                            Ex. 1-8                                                                           Ex. 1-9                             __________________________________________________________________________    Thickness of Upper                                                                       0.5 0.5  0.5 0.5  0.5 0.5  0.5 0.5                                 Magnetic Layer (μm)                                                        Thickness of Lower Non-                                                                  2   2    2   2    2   2    2   2                                   magnetic Layer (μm)                                                        Coating Method (%)                                                                       Multi-                                                                            Multi-                                                                             Multi-                                                                            Multi-                                                                             Multi-                                                                            Multi-                                                                             Multi-                                                                            Multi-                                         coating                                                                           coating                                                                            coating                                                                           coating                                                                            coating                                                                           coating                                                                            coating                                                                           coating                             Evaluation                                                                    MHz Output Power (dB)                                                                    1.5 6.5  5.9 6.1  --  6.2  6   1.5                                 C/N Ratio, (dB)                                                                          2   6    6.2 6    --  5.9  6.3 2                                   Still Durability                                                                         60 min.                                                                           60 min.                                                                            60 min.                                                                           60 min.                                                                            --  25 min.                                                                            60 min.                                                                           60 min.                                        or more                                                                           or more                                                                            or more                                                                           or more       or more                                                                           or more                             Running Durability                                                                       G   B    G   G    --  B    G   G                                   __________________________________________________________________________     *Coating Method: Examples 11 to 18 and Comparative Examples 11 to 16 and      18 to 19; Simultaneous multicoating                                           Comparative Example 18; Successive mulicoating                           

As is apparent from the results of Tables 1 to 3 above, the samples ofthe present invention have high reproduction output power and C/N andhave excellent still durability and running durability. The sample ofComparative Example 1-1, containing an inorganic powder having a largersize than the claimed defined range as the powder (A), has poorelectromagnetic characteristics. The sample of Comparative Example 1-2,containing a carbon black having a larger size than the claimed range,also has poor electromagnetic characteristics. The sample of ComparativeExample 1-3, containing powder (C) having a large size, also has poorelectromagnetic characteristics. The sample of Comparative Example 1-4,containing powder (A) having a small size, also has poor electromagneticcharacteristics. The sample of Comparative Example 1-5, containing thepowder (B) having a small size, also has poor electromagneticcharacteristics. The sample of Comparative Example 1-6, containing thepowder (C) having a small size, also has poor running durability. Thesample of Comparative Example 1-7, as being formed by successive coatingof coating the upper layer after the previously coated lower layer hasbeen dried, could not be evaluated by the determined methods. The sampleof Comparative Example 1-8, not containing the powder (C), did not haveimproved still durability and running durability. The sample ofComparative Example 1-9, not containing the powder (A), has poorelectromagnetic characteristics.

As explained in detail in the above, since the lower non-magnetic layerconstituting the magnetic recording medium of the present invention ismade of particularly selected powder composition comprising particularpowder components each of a specifically defined amount, the medium hasimproved still durability and running durability and has highreproduction output and C/N ratio. The medium therefore has an extremelythin magnetic layer as coated over the non-magnetic layer, which iscomparable to a magnetic recording medium having a thin ferromagneticmetal layer coated on the support. The producibility of the magneticrecording medium of the present invention is excellent.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

EXAMPLE 2

The samples in Examples 2-1 to 2-6 and Comparative Examples 2-1 to 2-4were prepared in the same manner as in Example 1, except that thefactors as indicated Table 4 below were used. These samples were testedand evaluated in the same manner as in Example 1, and the resultsobtained are shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________                         Comp.     Comp.                Comp.                                                                              Comp.                           Ex. 2-1                                                                            Ex. 2-2                                                                            Ex. 2-1                                                                            Ex. 2-3                                                                            Ex. 2-2                                                                            Ex. 2-4                                                                             Ex. 2-5                                                                            Ex. 2-6                                                                            Ex.                                                                                Ex.                  __________________________________________________________________________                                                             2-4                  Powder A                                                                      kind       α-Fe.sub.2 O.sub.3                                                           α-Fe.sub.2 O.sub.3                                                           α-Fe.sub.2 O.sub.3                                                           TiO.sub.2                                                                          TiO.sub.2                                                                          α-Fe.sub.2 O.sub.3                                                            α-Fe.sub.2 O.sub.3                                                           α-Fe.sub.2 O.sub.3                                                           α-Fe.sub.2                                                              O.sub.3                                                                            α-Fe.sub.2                                                              O.sub.3              specific gravity                                                                         5.1  5.1  5.1  4.2  4.2  5.1   5.1  5.1  5.1  5.1                  shape      acicular                                                                           acicular                                                                           acicular                                                                           acicular                                                                           acicular                                                                           tabular                                                                             tabular                                                                            tabular                                                                            tabular                                                                            tabular              long axis or                                                                             0.2  0.25 0.3  0.065                                                                              0.04 0.2   0.3  0.02 0.35 0.008                tabular size (μm)                                                          aspect ratio                                                                             10   12   13   4    2.5  8     12   3    22   2                    content, as weight ratio                                                                 75   75   75   75   75   75    75   75   75   75                   volume ratio (%)                                                                         56.3 56.3 56.3 61.0 61.0 56.3  56.3 56.3 56.3 56.3                 Powder B                                                                      mean particle size (μm)                                                               0.016                                                                              0.016                                                                              0.016                                                                              0.016                                                                              0.016                                                                              0.016 0.016                                                                              0.016                                                                              0.016                                                                              0.016                content, as weight ratio                                                                 15   15   15   15   15   15    15   15   15   15                   volume ratio (%)                                                                         34.2 34.2 34.2 30.5 30.5 34.2  34.2 34.2 34.2 34.2                 Powder C                                                                      kind       α-Al.sub.2 O.sub.3                                                           α-Al.sub.2 O.sub.3                                                           α-Al.sub.2 O.sub.3                                                           α-Al.sub.2 O.sub.3                                                           α-Al.sub.2 O.sub.3                                                           α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                           α-Al.sub.2 O.sub.3                                                           α-Al.sub.2                                                              O.sub.3                                                                            α-Al.sub.2                                                              O.sub.3              specific gravity                                                                         4    4    4    4    4    4     4    4    4    4                    mean particle size (μm)                                                               0.2  0.2  0.2  0.2  0.2  0.2   0.2  0.2  0.2  0.2                  shape      cubic                                                                              cubic                                                                              cubic                                                                              cubic                                                                              cubic                                                                              cubic cubic                                                                              cubic                                                                              cubic                                                                              cubic                content, as weight ratio                                                                 10   10   10   10   10   10    10   10   10   10                   volume ratio (%)                                                                         9.6  9.6  9.6  8.5  8.5  9.6   9.6  9.6  9.6  9.6                  __________________________________________________________________________                         Comp.     Comp.                Comp.                                                                              Comp.                           Ex. 2-1                                                                            Ex. 2-2                                                                            Ex. 2-1                                                                            Ex. 2-3                                                                            Ex. 2-2                                                                            Ex. 2-4                                                                             Ex. 2-5                                                                            Ex. 2-6                                                                            Ex.                                                                                Ex.                  __________________________________________________________________________                                                             2-4                  Thickness of Upper                                                                       0.5  0.5  0.5  0.5  0.5  0.5   0.5  0.5  0.5  0.5                  Magnetic Layer (μm)                                                        Thickness of Lower Non-                                                                  2    2    2    2    2    2     2    2    2    2                    magnetic Layer (μm)                                                        Coating Method                                                                           Simulta-                                                                           Simulta-                                                                           Simulta-                                                                           Simulta-                                                                           Simulta-                                                                           Simulta-                                                                            Simulta-                                                                           Simulta-                                                                           Simulta-                                                                           Simulta-                        neous                                                                              neous                                                                              neous                                                                              neous                                                                              neous                                                                              neous neous                                                                              neous                                                                              neous                                                                              neous                           multi-                                                                             multi-                                                                             multi-                                                                             multi-                                                                             multi-                                                                             multi-                                                                              multi-                                                                             multi-                                                                             multi-                                                                             multi-                          coating                                                                            coating                                                                            coating                                                                            coating                                                                            coating                                                                            coating                                                                             coating                                                                            coating                                                                            coating                                                                            coating              Evaluation                                                                    7 MHz Output (dB)                                                                        7.2  6.5  4.1  7.8  8    6.1   5.8  6.5  6.6  1.2                  C/N Ratio (dB)                                                                           6.8  6.2  3.5  7.5  7.5  6.2   6    7    7    0                    Still Durability                                                                         60 min.                                                                            60 min.                                                                            60 min.                                                                            60 min.                                                                            50 min.                                                                            60 min.                                                                             60 min.                                                                            60 min.                                                                            32 min.                                                                            60 min.                         or more                                                                            or more                                                                            or more                                                                            or more   or more                                                                             or more                                                                            or more   or more              Running Durability                                                                       G    G    G    G    B    G     G    G    B    G                    __________________________________________________________________________

As is apparent from the results shown in Table 4, the sample ofComparative Example 2-1, containing an acicular powder having a largerlong axis of 0.3 μm than the claimed defined range of from 0.05 to 0.25μm, has deteriorated 7 MHz output and C/N ratio. The sample ofComparative Example 2-2, containing a smaller long axis of 0.04 μm thanthe claims defined range and a smaller aspect ratio of 2.5 than theclaimed defined range of from 3 to 15, has deteriorated still durabilityand running durability. On the other hand, the sample of ComparativeExamples 2-3, containing a tabular powder having a larger tabular sizeof 0.35 μm than the claimed defined range of 0.01 to 0.30 μm and alarger aspect ratio of 22 than the claimed defined range of 3 to 20, hasdeteriorated still durability and running durability. The sample ofComparative Example 2-4, containing a tabular powder having a smallertabular size of 0.008 μm and a smaller aspect ratio of 2 than theclaimed defined ranges, has deteriorated 7 MHz output and C/N ratio.

What is claimed is:
 1. A magnetic recording medium containing a supporthaving thereon in order a non-magnetic layer unit and a magnetic layerunit, said magnetic recording medium having been produced by providing anon-magnetic layer unit containing at least one non-magnetic layer andcomprising a non-magnetic powder dispersed in a binder on a non-magneticsupport followed by providing, while the non-magnetic layer is stillwet, a magnetic layer unit containing at least one magnetic layer andcomprising a ferromagnetic powder dispersed in a binder on thenon-magnetic layer, wherein the mean thickness of the magnetic layer is1.0 μm or less and the non-magnetic powder in the non-magnetic layercomprises (A) a tabular inorganic non-magnetic powder having a tabularsize of from 0.01 to 0.3 μm and an aspect ratio of from 3 to 20, (B) acarbon black powder having a mean particle size of from 0.01 to 0.04 μmand (C) a powder component having a larger mean particle size than themean particle size of each of powders (A) and (B).
 2. The magneticrecording medium as claimed in claim 1, wherein the powder (C) comprisesa granular or polyhedral powder having a mean particle size of from 0.07μm to less than 1 μm.
 3. The magnetic recording medium as claimed inclaim 1, wherein the powder (C) comprises an acicular powder having amean length in the long axis of from 0.05 μm to less than 1 μm and anaverage aspect ratio of 10 or more.
 4. The magnetic recording medium asclaimed in claim 1, wherein the powder (C) comprises a tabular powderhaving a mean tabular diameter of from 0.1 μm to less than 2 μm and anaverage aspect ratio of 10 more.
 5. The magnetic recording medium asclaimed in claim 1, wherein respective volume proportions of the powders(A), (B) and (C) to the total volume of non-magnetic powders present inthe lower non-magnetic layer is 40 to 80% of powder (A), 15 to 40% ofpowder (B) and 2 to 26% of powder (C).
 6. The magnetic recording mediumas claimed in claim 1, wherein the support has a thickness of 1 to 100μm, the lower non-magnetic layer has a mean thickness of 1 to 5 μm andthe upper magnetic layer has a mean thickness of 0.05 to 1.0 μm.
 7. Themagnetic recording medium as claimed in claim 1, wherein the powder (C)has a Mohs' hardness of 4 or more and a specific gravity of from 2 to 6.8. The magnetic recording medium as claimed in claim 1, wherein thecarbon black of powder (B) has a specific surface area of from 100 to500 m² /g, a DBP absorption of from 20 to 400 ml/g, a pH value of from 2to 10, a water content of from 0.1 to 10% and a tap density of from0.1.to 1 g/cc.
 9. The magnetic recording medium as claimed in claim 1,wherein the tabular non-magnetic powder mainly comprises α-Fe₂ O₃.
 10. Amagnetic recording medium containing a support having thereon in order anon-magnetic layer unit and a magnetic layer unit, said magneticrecording medium having been produced by providing a non-magnetic layerunit containing at least one non-magnetic layer and comprising anon-magnetic powder dispersed in a binder on a non-magnetic supportfollowed by providing, while the magnetic layer is still wet, a magneticlayer unit containing at least one magnetic layer and comprising aferromagnetic powder dispersed in a binder on the non-magnetic layer,wherein the mean thickness of the magnetic layer is 1.0 μm or less andthe non-magnetic powder in the non-magnetic layer comprises (A) anacicular inorganic non-magnetic powder having a long axis of from 0.05to 0.25 μm and an aspect ratio of from 3 to 15, (B) a carbon blackpowder having a mean particle size of from 0.01 to 0.04 μm and (C) apowder component having a larger mean particle size than the meanparticle size of each of powders (A) and (B).
 11. The magnetic recordingmedium as claimed in claim 10, wherein the acicular non-magnetic powdermainly comprises α-Fe₂ O₃ or TiO₂.
 12. The magnetic recording medium asclaimed in claim 10, wherein the powder (C) comprises a granular orpolyhedral powder having a mean particle size of from 0.07 μm to lessthan 1 μm.
 13. The magnetic recording medium as claimed in claim 10,wherein the powder (C) comprises an acicular powder having a mean lengthin the long axis of from 0.05 μm to less than 1 μm and an average aspectratio of 10 or more.
 14. The magnetic recording medium as claim 12,wherein the powder (C) comprises a tabular powder having a mean tabulardiameter of from 0.1 μm to less than 2 μm and an average aspect ratio of10 more.
 15. The magnetic recording medium as claimed in claim 10,wherein respective volume proportions of the powders (A), (B) and (C) tothe total volume of non-magnetic powders present in the lowernon-magnetic layer is 40 to 80% of powder (A), 15 to 40% of powder (B)and 2 to 26% of powder (C).
 16. The magnetic recording medium as claimedin claim 10, wherein the support has a thickness of 1 to 100 μm, thelower non-magnetic layer has a mean thickness of 1 to 5 μm and the uppermagnetic layer has a mean thickness of 0.05 to 1.0 μm.
 17. The magneticrecording medium as claimed in claim 10, wherein the powder (C) has aMohs' hardness of 4 or more and a specify gravity of from 2 to
 6. 18.The magnetic recording medium as claimed in claim 10, wherein the carbonblack powder (B) has a specific surface area of from 100 to 500 m² /g, aDBP absorption of from 20 to 400 ml/g, a pH value of from 2 to 10, awater content of from 0.1 to 10% and a tap density of from 0.1 to 1g/cc.